Commercially, nitroaromatics, and particularly dinitrotoluene, are produced by the mixed acid nitration of toluene, the mixed acid being a mixture of concentrated sulfuric and concentrated nitric acid. In the production of dinitrotoluene process, for example, toluene is first nitrated to form mononitrotoluene and then separated from the spent acid aqueous phase. The crude mononitrotoluene is then dinitrated with fresh acid in a second nitration stage. As is known the dinitrotoluene product recovered from the dinitration reactor contains impurities, primarily nitrophenolics, such as nitrocresol and picric acid.
Traditionally, it has been common practice to remove the nitrophenolic materials from the organic dinitrotoluene phase because it has been believed they adversely affect the performance of hydrogenation catalysts in the reduction of dinitrotoluene to form toluenediamine. Removal of nitrophenolic material from the dinitrotoluene reaction product has been achieved by contacting that product with alkaline materials to convert the nitrophenolic materials with the crude dinitrotoluene reaction product to water soluble salts. The water soluble salts then are discharged.
Recent environmental regulations have placed severe restrictions on the discharge of aqueous streams containing alkali metal salts of nitrophenolic materials. As is known these materials are not readily subject to biodegradation and then there is an unknown factor regarding the toxicity of the materials in the amounts that would normally be discharged to the environment. Therefore it is desired that techniques be developed to remove nitrophenolic materials from a dinitrotoluene reaction product without creating an environmentally unacceptable aqueous discharge stream.
U.S. Pat. No. 4,482,769, although not prior art to this application, discloses a process for separating trinitroorthocresol from a reaction product while leaving dinitroorthocresol in the dinitrotoluene product. The process involves selectively precipitating the dinitroorthocresol from an aqueous stream by contacting with alkaline material.
Patents which show the removal of nitrophenolic material from crude dinitrotoluene streams by the addition of alkaline material are British Pat. No. 1,031,450; and U.S. Pat. Nos. 4,224,249; 4,361,712 and 4,230,567. Only the '567 patent addresses the problem of disposal of the wastewater streams containing alkali metal salts of nitrophenolic material. As acknowledged in that patent, direct incineration of the wastewater stream is considered to be energy intensive and is unacceptable for that reason. The approach taken in the '567 patent involves a degradation process as opposed to a combustion process.